1. Field of the Invention
The present invention relates to a digital signal record apparatus which is useful in a digital video tape recorder (hereinafter, referred to as a "a digital VTR"), a digital disk player, or the like having a track format that records digital video and audio signals in respective predetermined areas in a slant track, and which receives digital video and audio signals in the form of a bitstream and records the bitstream, and also to a digital signal playback apparatus which plays back a record medium on which recording was conducted by the digital signal record apparatus.
2. Description of Related Art
FIG. 1 is a diagram showing a track pattern employed in a common home digital VTR of the prior art. As shown, slant tracks are formed on a magnetic tape, and each track is divided into two areas, a video area for recording a digital video signal and an audio area for recording a digital audio signal.
There are two methods to record video and audio signals on such home digital VTRs. One is the so-called baseband recording method in which the video and audio signals are inputted in analog form, and recorded in digital form after performing a high-efficiency coding to reduce data rate; the other is the so-called transparent recording method in which bitstreams transmitted in digital form are recorded.
For recording of the Advanced Television (ATV) signal currently under consideration in the United States, or the Digital Video Broadcasting (DVB) signal currently under investigation in Europe, the latter method, i.e., the transparent recording method, is suitable. Major reasons are that the ATV signal or the DVB signal is already digital-compressed and does not require high-efficiency encoders or decoders, and that there occurs no degradation in picture quality since the signal is recorded directly. On the other hand, the major drawback is inferior picture quality in trick playback modes such as high-speed playback, still-motion playback, and slow motion playback. In particular, by simply recording bitstreams directly on slant tracks, useful pictures cannot be reproduced in high-speed playback.
One digital VTR method for recording the ATV signal was proposed in a technical report "A Recording Method of ATV data on a Consumer Digital VCR" presented at "International Workshop on HDTV '93" held from Oct. 26 to 28 in Ottawa, Canada. This report will be used as the prior art in the following description.
According to the basic specification of a prototype home digital VTR, assuming the recording rate of the digital video signal is 25 Mbps and the field frequency is 60Hz, one video frame is recorded in video areas on 10 tracks in standard definition (SD) mode. Here, if the data rate of the ATV signal is 17 to 18Mbps. Transparent recording of the ATV signal is possible in this SD mode.
FIGS. 2A and 2B are diagrams showing head scan paths in normal playback and high-speed playback on a digital VTR. As shown, adjacent slant tracks are recorded alternately across the tape by rotary heads having different azimuth angles. In normal playback, the tape transport speed is the same as in recording, so that the rotary heads precisely trace the recorded tracks as shown in FIG. 2A. In high-speed playback, on the other hand, since the tape speed is different, the heads move across several tracks, each head thus being able to play back only fragments of the same azimuth tracks. FIG. 2B shows an example in fast forward playback at the speed five times the normal speed.
In MPEG2 bitstreams (bitstreams of the ATV signal and the DVB signal are substantially compatible with MPEG2 bitstreams), only intracoded blocks can be decoded independently without referencing other frames. If an MPEG2 bitstream is recorded on tracks in successive order, in high-speed playback, intra-coded data is taken out of a playback signal by intermittent playback and an image is reconstructed only the the intra-coded data. In this case, on the screen the reproduced areas will be non-continuous, and fragments of blocks will be dispersed across the screen. Furthermore, since the bitstream is variable-length encoded, there is no guarantee that the whole screen will be updated periodically, and there is a possibility that some portions may remain unupdated for long periods of time. As a result, the picture quality in high-speed playback will not be sufficient and unacceptable for a home digital VTR.
FIG. 3 is a block diagram showing a bitstream recording apparatus of the prior art capable of performing a high-speed playback. Here, the video area on each track is divided into a main area, where the whole bitstream of the ATV signal is recorded, and a duplication area, where portions of the bitstream critical for image reconstruction in high-speed playback are recorded (such portions are hereinafter referred to as High Priority (HP) data). Since only intra-coded blocks are valid in high-speed playback, intra-coded blocks are recorded in the duplication area; to further reduce the data amount, low-frequency components are extracted from all the intra-coded blocks and recorded as HP data. In FIG. 3, reference numeral 301 is a bitstream input terminal, 302 is a bitstream output terminal, 303 is an HP data output terminal, 304 is a variable-length decoder, 305 is a counter, 306 is a data extraction circuit, and 307 is an EOB (end of block) appending circuit.
An MPEG2 bitstream is inputted via the input terminal 301 and outputted unprocessed via the output terminal 302 for recording successively in the main area. The bitstream inputted via the input terminal 301 is also fed to the variable-length decoder 304, which analyzes the syntax of the MPEG2 bitstream and detects an intra-image; in response, the counter 305 generates the timing at which the data extraction circuit 306 extracts the low-frequency components of each block of the intra-image. Then, the EOB appending circuit 307 appends an EOB to construct HP data which is recorded in the duplication area.
FIG. 4 is a diagram conceptually showing normal playback and high-speed playback in a prior art digital VTR. In normal playback, the whole bitstream recorded in the main area is reproduced and supplied to an MPEG2 decoder external to the digital VTR. The HP data is discarded. On the other hand, in high-speed playback, only the HP data recorded in the duplication area is selected and sent to the decoder, while the bitstream from the main area is discarded.
Next, the arrangement of the main area and duplication area on each track will be described. FIG. 5 is a diagram showing an example of the scan path of the rotary head in common high-speed playback. If the tape speed is an integral multiple of the normal speed and phase-lock controlled, the head scanning is synchronized with the tracks with the same azimuth, reproducing data always from the same positions. In FIG. 5, if portions of the reproduced signal whose output levels are higher than -6 dB are played back, hatched regions will be played back by one head. FIG. 5 shows an example of high-speed playback 9 times the normal speed. At this 9-times playback speed, it is guaranteed that the signal will be read from the hatched regions. It can therefore be seen that the HP data should be recorded in these areas. However, at other fast playback speeds, there is no guarantee that the signal will be read; the regions need to be set so that the signal can be read at different tape speeds.
FIG. 6 is a diagram showing overlapped areas in plural types of conventional high-speed playback, and it shows examples of scan areas of three types of tape speed in which the rotary head is synchronized with the same azimuth track. Some of the regions scanned at various tape speeds overlap. Duplication areas are selected from among these regions to guarantee HP data reading at different tape speeds. Shown in FIG. 6 are examples of fast playback 4, 9, and 17 times the normal speed. The scan areas shown are the same as those that will be selected for fast playback -2, -7, and -15 times the normal speed.
It is not possible for the rotary head to trace exactly the same regions at different tape speeds, because the number of tracks that the rotary head crosses is different at different tape speeds. Furthermore, it is required that the scan areas be traced from any of the same azimuth tracks. FIG. 7 is a diagram showing examples of scan paths of the rotary head in 5-times speed and 9times speed in a prior art digital VTR. In FIG. 7, regions 1, 2 and 3 are selected from among the overlapped regions between 5-times and 9times speeds. By repeating the same HP data on nine tracks, the HP data can be read at both 5-times and 9-times speeds.
FIG. 8 is a diagram showing examples of scan paths of two rotary heads in 5-times speed playback in the prior art digital VTR. As can be seen from the figure, by repeating the same HP data over the same number of tracks as the speed expressed as a multiple of the normal speed, the HP data can be read by the rotary head synchronized with the same azimuth tracks. As a result, by duplicating the HP data over the same number of tracks as the maximum tape speed in high-speed playback expressed as a multiple of the normal speed, reading of the duplicate HP data can be guaranteed at different tape speeds in both forward and reverse directions.
FIG. 9 shows a track format employed in the prior art digital VTR; shown here is an example of a main and duplication area layout. In a home digital VTR, the video area on each track consists of 135 sync blocks; in the example shown, the main area consists of 97 sync blocks and the duplication area consists of 32 sync blocks. This duplication area is made up of the overlapped areas between the 4-times, 9-times, and 17-times speeds shown in FIG. 6. In this case, the data rate of the main area is about 17.46 Mbps, and that of the duplication area is about 338.8 kbps since identical data is recorded 17 times.
In a prior art home digital VTR which is configured as described above, trick playback data are recorded several times in an overlap manner in a duplication area. Consequently, there arises a problem in that the record rate of trick playback data is very low and, particularly in slow motion playback or high-speed playback, a played back image fails to have a sufficient picture quality. When intra-frame is 2 frames per second, for example, the data amount with respect to only intra-frame coding of the ATV signal is estimated to be about 3 Mbps. In the prior art example, however, the amount of data which can be recorded is a small as about 340 kbps, so that the picture quality of a played back image is extremely degraded.
FIGS. 10A and 10B are data format diagrams showing the configuration of error correcting codes in a video signal area and an audio signal area in 1 track of a digital VTR according to the SD standarde defined by the SD mode. According to the SD standard, as the error correcting code in the video signal area, (85, 77, 9) Reed-Solomon code (hereinafter, referred to as "C1 check code") is used in the recording direction, and (149, 138, 12) Reed-Solomon code (hereinafter, referred to as "C2 check code") is used in the vertical direction. As the error correcting code in the audio signal area, (85, 77, 9) Reed-Solomon code (C1 check code ) same as that for a video signal is used in the recording direction, and (14, 9, 6) Reed-Solomon code (hereinafter, referred to as "C3 check code") is used in the vertical direction.
One sync block according to the SD standard is shown in FIG. 11. According to the SD standard, as shown in FIG. 11, data of 1 sync block are configured by 90 bytes. In the block, a sync pattern and an ID signal are recorded in the first 5 bytes, and an error correcting code (C1 check code) is recorded in the last 8 bytes.
The ATV signal is subjected to data compression by using a compression method based on motion compensative predication. The compressed data are configured by intra-data (intrafield or intra-frame coding) which allow an image to be reconstructed by using only playback data, and inter-data (interfield or inter-frame coding) which allow an image to be reconstructed by using data of the reference field (or frame) and playback data. In the ATV signal, therefore, when an error occurs in playback data, the error propagates over plural fields or frames, resulting in a playback image which is visually very inferior. When a digital VTR according to the SD standard is to be used as a storage medium for storing data, program, etc. for a computer system or the like, it is desired to append a stronger error correcting code in order to reconstruct (error-correct) data which were not played back by a drop-out due to a scratch on a magnetic tape, dust adhering to the magnetic tape, or the like.
In trick playback such as high-speed playback, slow motion playback, or still-motion playback, the rotary head obliquely crosses record tracks so that signals are intermittently played back from the tracks. In trick playback, therefore, it is impossible to configure an error correcting block (video data) such as shown in FIG. 10A. In trick playback, consequently, only the error correcting due to the C1 check code is conducted on playback data.
In the case where only the error correction due to the C1 check code is conducted, when the symbol error rate is 0.01, the probability of error detection is 1.56.times.10.sup.-3, or one error is detected in every about 8 sync blocks. In trick playback, particularly, it is often that the symbol error rate is 0.01 or higher because the playback output level is not stabilized. Recorded data have undergone variable-length coding. When an error once occurs, therefore, it is impossible to use subsequent playback data, thereby degrading the picture quality of a played back image. Furthermore, the residual error rate is as high as 7.00.times.10.sup.-8 and the error occurrence frequency is very high.
In the DVB signal, the record rate is varied depending on programs. Specifically, when the picture quality similar to that of the PAL system or the SECAM system which is currently used is to be attained, the record rate is set to be about 5 to 5.5 Mbps, and, when the picture quality similar to "studio quality" is to be attained, the record rate is set to be about 9 Mbps. When a record signal having plural record rates is recorded into a conventional digital VTR, there arise the following problems. In the case where a program of 9 Mbps is recorded into a conventional digital VTR, for example, nothing is recorded in an area of about 8.5 Mbps because the record rate of the main area is 17.46 Mbps as described above, resulting in a very low usage efficiency of a magnetic tape.